The present invention relates to multiple access protocols for a wireless communication system and, more particularly, to a contention-based multiple access protocol for an uplink channel having a plurality of subcarriers.
In a wireless packet data network, a plurality of user terminals transmits packet data to an access point over a shared uplink channel. A multiple access protocol is used to share the uplink channel among the user terminals. The random access protocol may be a reservation based protocol or a contention based protocol. In either case, the channel is typically divided in the time domain into a sequence of time slots. The user terminals share the channel by transmitting in different time slots. The channel may also be divided in the frequency domain, as in the case of Orthogonal Frequency Division Multiplexing (OFDM) systems. In such cases, multiple user terminals may transmit in the same timeslot, but on different subcarrier frequencies.
A reservation-based random access protocol reserves resources for individual user terminals. The mobile stations request permission from the access point to transmit on the shared uplink channel. If the request is granted, the access point reserves resources for the user terminal and sends a grant message to the user terminal identifying the reserved resources. The reserved resources may, for example, comprise a timeslot or portion of a timeslot. After receiving permission, the user terminal transmits its data using the allocated resources. One shortcoming of reservation-based multiple access protocols is the delay incurred in the request/grant procedure.
In contention-based multiple access protocol, resources are not reserved and the user terminals compete with one another for access to the channel. One such protocol is called slotted ALOHA. In slotted ALOHA, the shared uplink channel is divided into a sequence of time slots. When a user terminal has packet data to transmit, it selects a time slot and begins its transmission at the start of the selected time slot. With single packet reception, the packet will be received by the access point if no other user terminal transmits in the same slot. However, if another user terminal transmits in the same time slot, a collision occurs and neither packet will be received. In the event of a collision, each user terminal backs off a random amount and retransmits in another time slot. It has been shown that the maximum throughput using slotted ALOHA is 0.36 packets per slot. This low throughput is the main disadvantage of the slotted ALOHA approach.
Multi-packet reception may be used to improve the throughput of the slotted ALOHA approach. With multi-packet reception, the access point employs multi-user detection techniques to detect the transmissions from multiple user terminals in the same time slot. Multi-packet reception may significantly increase the throughput of the slotted ALOHA approach.
One approach to slotted ALOHA with multi-packet reception is disclosed in a co-pending application titled “Arrangement and Method for Contention-Based Multiaccess in a Wireless Communication System.” In this application, slotted ALOHA and multi-packet reception are applied to share an uplink channel in an orthogonal frequency division multiplexing (OFDM) system. The uplink channel comprises a plurality of subcarriers. The access point is equipped with multiple receive antennas. There are a plurality of user terminals, each of which has a single antenna. The shared uplink channel is divided into a series of time slots. When a user terminal has packet data to send, the user terminal selects a timeslot and starts transmitting at the beginning of the selected time slot. A slow power control mechanism is employed so that the average power spectral density of the received signals from all user terminals is the same at the access point. The access point reports the noise level to the user terminals, which use the noise level to determine their data transmission rate. Using multi-user detection techniques, the access point is able to demodulate and decode the transmissions from multiple user terminals in a single time slot.
One limitation of the above-described technique is that each user terminal must transmit on the same subcarriers. This limitation avoids ambiguity over which subcarriers to demodulate when receiving signals from multiple user terminals. As long as the access point can detect the identity of the user terminal, the access point can demodulate and decode the signals from multiple user terminals.
It is not always advantageous for mobile terminals to use the same set or number of subcarriers when transmitting on the uplink channel. Packet sizes may vary from one user terminal to another. A user terminal with a small packet does not need as many subcarriers to transmit the packet as compared to a user terminal with a larger packet. Multiple access interference could be reduced if a user terminal could elect to use less than all of the subcarriers to transmit a packet. Additionally, a user terminal with a large path loss may not have sufficient power available to transmit on all subcarriers and maintain the average power spectral density on all subcarriers.
It would be advantageous to have a contention-based multiple access approach that allowed the user terminal to autonomously select the number of subcarriers to use for transmissions on the uplink. Such a system would accommodate a variable data rate, reduce overall interference in the system, and avoid power limitations in circumstances where user terminals have a large path loss.